Planetary gear train of automatic transmission for vehicles

ABSTRACT

Ten or more forward speeds and at least four reverse speeds are achieved by a planetary gear train of an automatic transmission for a vehicle including an input shaft, an output shaft, four planetary gear sets respectively having three rotational elements, and six control elements for selectively interconnecting the rotational elements and a transmission housing.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2016-0031471 filed in the Korean IntellectualProperty Office on Mar. 16, 2016, the entire contents of which areincorporated herein by reference.

BACKGROUND

(a) Field of the Invention

The present invention relates to an automatic transmission for avehicle, more particularly, to a planetary gear train of the automatictransmission capable of realizing at least ten forward speeds, therebyimproving power delivery performance and fuel consumption due tomulti-stages, and improving driving stability by utilizing a lowrotation speed of an engine.

(b) Description of the Related Art

Generally, in automatic transmissions, achieving more shift stages canmaximize fuel consumption and driving efficiency.

In particular, engine research has been undertaken to achieve weightreduction and to enhance fuel consumption by so-called downsizing, andresearch on an automatic transmission has been performed tosimultaneously provide better drivability and fuel consumption byachieving more shift stages.

In order to achieve more shift stages for an automatic transmission, thenumber of parts, particularly the number of planetary gear sets istypically increased and installability, production cost, weight, and/orpower flow efficiency according to total length of transmission may beincreased.

Therefore, in order to maximize fuel consumption of an automatictransmission having more shift stages, it is important for betterefficiency to be derived by a smaller number of parts.

In this respect, an eight-speed automatic transmission has been recentlyintroduced, and a planetary gear train for an automatic transmissionenabling more shift stages is continuously required.

However, the majority of general automatic transmissions having morethan eight speeds include three to four planetary gear sets and five tosix control elements (friction elements), in this case, total length isincreased, which has drawbacks of deteriorating installability.

As a result, plural rows structures of planetary gear sets have beenadopted, or a dog clutch is applied in place of wet control elements.However, in this case, applicable structure is restricted and shift feelis deteriorated by applying the dog clutch.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY

The present invention provides a planetary gear train of an automatictransmission for a vehicle having advantages of obtaining shift-stagesof at least ten forward speeds and at least four reverse speeds by useof a minimal number of parts, improving power delivery performance andfuel consumption by multi-stages of an automatic transmission, andimproving driving stability of a vehicle by utilizing a low rotationspeed of an engine.

A planetary gear train according to an exemplary embodiment of thepresent invention includes an input shaft for receiving an enginetorque; an output shaft for outputting a shifted torque; a firstplanetary gear set having first, second, and third rotational elements;a second planetary gear set having fourth, fifth, and sixth rotationalelements; a third planetary gear set having seventh, eighth, and ninthrotational elements; a fourth planetary gear set having tenth, eleventh,and twelfth rotational elements; a first shaft interconnecting the firstrotational element and the fourth rotational element; a second shaftconnected with the second rotational element; a third shaftinterconnecting the third rotational element and the seventh rotationalelement; a fourth shaft connected with the fifth rotational element andselectively connected with the second shaft and directly connected withthe input shaft; a fifth shaft connected with the sixth rotationalelement and selectively connected with the second shaft; a sixth shaftinterconnecting the eighth rotational element and the twelft rotationalelement and selectively connected with the fifth shaft; a seventh shaftinterconnecting the ninth rotational element and the eleventh rotationalelement and directly connected with the output shaft; and an eighthshaft connected with the tenth rotational element and selectivelyconnected with the fourth shaft.

The first, third and sixth shafts are selectively connected with thetransmission housing respectively.

The first, second, and third rotational elements of the first planetarygear set may be respectively a first sun gear, a first planet carrier,and a first ring gear of the first planetary gear set. The fourth,fifth, and sixth rotational elements of the second planetary gear setmay be respectively a second sun gear, a second planet carrier, and asecond ring gear of the second planetary gear set. The seventh, eighth,and ninth rotational elements of the third planetary gear set may berespectively a third sun gear, a third planet carrier, and a third ringgear of the third planetary gear set. The tenth, eleventh, and twelfthrotational elements of the fourth planetary gear set may be respectivelya fourth sun gear, a fourth planet carrier, and a fourth ring gear ofthe fourth planetary gear set.

A planetary gear train according to an exemplary embodiment of thepresent invention may further include a first clutch selectivelyconnecting the second shaft and the fourth shaft; a second clutchselectively connecting the fourth shaft and the eighth shaft; a thirdclutch selectively connecting the second shaft and the fifth shaft; afourth clutch selectively connecting the fifth shaft and the sixthshaft; a first brake selectively connecting the first shaft and thetransmission housing; a second brake selectively connecting the thirdshaft and the transmission housing and a third brake selectivelyconnecting the sixth shaft and the transmission housing.

According to an exemplary embodiment of the present invention,shift-stages of at least ten forward speeds and at least four reversespeeds may be realized by combination of four planetary gear sets ofsimple planetary gear sets and six control elements.

In addition, a planetary gear train according to an exemplary embodimentof the present invention may realize shift stages appropriate forrotation speed of an engine due to multi-stages of an automatictransmission and improve driving stability of a vehicle by utilizing alow rotation speed of an engine.

In addition, a planetary gear train according to an exemplary embodimentof the present invention maximize engine driving efficiency bymulti-stages of an automatic transmission, and may improve powerdelivery performance and fuel consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

FIG. 2 is an operational chart for respective control elements atrespective shift stages in a planetary gear train according to anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. Throughout the specification, unless explicitly describedto the contrary, the word “comprise” and variations such as “comprises”or “comprising” will be understood to imply the inclusion of statedelements but not the exclusion of any other elements. In addition, theterms “unit”, “-er”, “-or”, and “module” described in the specificationmean units for processing at least one function and operation, and canbe implemented by hardware components or software components andcombinations thereof.

Further, the control logic of the present invention may be embodied asnon-transitory computer readable media on a computer readable mediumcontaining executable program instructions executed by a processor,controller or the like. Examples of computer readable media include, butare not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes,floppy disks, flash drives, smart cards and optical data storagedevices. The computer readable medium can also be distributed in networkcoupled computer systems so that the computer readable media is storedand executed in a distributed fashion, e.g., by a telematics server or aController Area Network (CAN).

Hereinafter, an exemplary embodiment of the present invention will bedescribed in detail with reference to drawings.

The drawings and description are to be regarded as illustrative innature and not restrictive, and like reference numerals designate likeelements throughout the specification.

In the following description, dividing names of components into first,second, and the like is to divide the names because the names of thecomponents are the same as each other and an order thereof is notparticularly limited.

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

Referring to FIG. 1, a planetary gear train according to an exemplaryembodiment of the present invention includes first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 arranged on a sameaxis, an input shaft IS, an output shaft OS, eight shafts TM1 to TM8interconnecting rotational elements of the first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4, four clutches C1 toC4 and three brakes B1, B2 and B3 as control elements, and atransmission housing H.

Torque input from the input shaft IS is shifted by cooperative operationof the first, second, third, and fourth planetary gear sets PG1, PG2,PG3, and PG4, and then output through the output shaft OS.

The simple planetary gear sets are arranged in the order of first,first, second, third and fourth planetary gear sets PG1, PG2, PG3 andPG4, from an engine side.

The input shaft IS is an input member and the torque from a crankshaftof an engine, after being torque-converted through a torque converter,is input into the input shaft IS.

The output shaft OS is an output member, and being arranged on a sameaxis with the input shaft IS, delivers a shifted torque to a drive shaftthrough a differential apparatus.

The first planetary gear set PG1 is a double pinion planetary gear set,and includes a first sun gear S1, a first planet carrier PC1 thatsupports a first pinion P1 externally engaged with the first sun gearS1, and a first ring gear R1 internally engaged with the first pinionP1. The first sun gear S1 acts as a first rotational element N1, thefirst planet carrier PC1 acts as a second rotational element N2, and thefirst ring gear R1 acts as a third rotational element N3.

The second planetary gear set PG2 is a single pinion planetary gear set,and includes a second sun gear S2, a second planet carrier PC2 thatsupports a second pinion P2 externally engaged with the second sun gearS2, and a second ring gear R2 internally engaged with the second pinionP2. The second sun gear S2 acts as a fourth rotational element N4, thesecond planet carrier PC2 acts as a fifth rotational element N5, and thesecond ring gear R2 acts as a sixth rotational element N6.

The third planetary gear set PG3 is a single pinion planetary gear set,and includes a third sun gear S3, a third planet carrier PC3 thatsupports a third pinion P3 externally engaged with the third sun gearS3, and a third ring gear R3 internally engaged with the third pinionP3. The third sun gear S3 acts as a seventh rotational element N7, thethird planet carrier PC3 acts as a eighth rotational element N8, and thethird ring gear R3 acts as a ninth rotational element N9.

The fourth planetary gear set PG4 is a single pinion planetary gear set,and includes a fourth sun gear S4, a fourth planet carrier PC4 thatsupports a fourth pinion P4 externally engaged with the fourth sun gearS4, and a fourth ring gear R4 internally engaged with the fourth pinionP4. The fourth sun gear S4 acts as a tenth rotational element N10, thefourth planet carrier PC4 acts as a eleventh rotational element N11, andthe fourth ring gear R4 acts as a twelfth rotational element N12.

In the arrangement of the first, second, third, and fourth planetarygear sets PG1, PG2, PG3, and PG4, the first rotational element N1 isdirectly connected with the fourth rotational element N4, the thirdrotational element N3 is directly connected with the seventh rotationalelement N7, the eighth rotational element N8 is directly connected withthe twelfth rotational element N12 and the ninth rotational element N9is directly connected with the eleventh rotational element N11 by eightshafts TM1 to TM8.

The eight shafts TM1 to TM8 are arranged as follows.

Each of the eight shafts TM1 to TM8 may be a rotational member thatinterconnects the input and output shafts and rotational elements of theplanetary gear sets PG1, PG2, PG3, and PG4, or may be a fixed memberfixed to the transmission housing H.

The first shaft TM1 directly connects the first rotational element N1(the first sun gear S1) and the fourth rotational element N4 (the secondsun gear S2), and is selectively connected with the transmission housingH, thereby acting as a selective fixed element.

The second shaft TM2 is connected with the second rotational element N2(the first planet carrier PC1).

The third shaft TM3 directly connects the third rotational element N3(the first ring gear R1) and the seventh rotational element N7 (thethird sun gear S3), and selectively connected with the transmissionhousing H, thereby acting as a selective fixed element.

The fourth shaft TM4 is directly connected with the fifth rotationalelement N5 (the second planet carrier PC2) and selectively connectedwith the second shaft TM2, and directly connected with the input shaftIS, thereby always acting as an input element.

The fifth shaft TM5 is connected with the sixth rotational element N6(second ring gear R2) and selectively connected with the second shaftTM2.

The sixth shaft TM6 directly connects the eighth rotational element N8(the third planet carrier PC3) and the twelfth rotational element N12(the fourth ring gear R4) and is selectively connected with the fifthshaft TM5, and is selectively connected with the transmission housing H,thereby acting as a selective fixed element.

The seventh shaft TM7 directly connects the ninth rotational element N9(third ring gear R3) and the eleventh rotational element N11 (fourthplanet carrier PC4), and directly connected with the output shaft OS,thereby always acting as an output element.

The eighth shaft TM8 is connected with the tenth rotational element N10(the fourth sun gear S4) and selectively connected with the fourth shaftTM4. That is, the eighth shaft TM8 is selectively connected with thefourth shaft TM4 which is directly connected with the input shaft IS,thereby acting as a selective fixed element.

The eight shafts TM1 to TM8, the input shaft IS, and the output shaft OSmay be selectively interconnected with one another by control elementsof four clutches C1, C2, C3, and C4.

The shafts TM1 to TM8 may be selectively connected with the transmissionhousing H, by control elements of three brakes B1, B2 and B3.

The four clutches C1 to C4 and the three brakes B1, B2 and B3 arearranged as follows.

The first clutch C1 is arranged between the second shaft TM2 and thefourth shaft TM4, so as to selectively connecting the second shaft TM2and the fourth shaft TM4 for power delivery.

The second clutch C2 is arranged between the fourth shaft TM4 and theeighth shaft TM8, so as to selectively connecting the fourth shaft TM4and the eighth shaft TM8 for power delivery.

The third clutch C3 is arranged between the second shaft TM2 and thefifth shaft TM5, so as to selectively connecting the second shaft TM2and the fifth shaft TM5 for power delivery.

The fourth clutch C4 is arranged between the fifth shaft TM5 and thesixth shaft TM6, so as to selectively connecting the fifth shaft TM5 andthe sixth shaft TM6 for power delivery.

The first brake B1 is arranged between the first shaft TM1 and thetransmission housing H, such that the first shaft TM1 may be selectivelyconnected with the transmission housing H and act as a fixed element.

The second brake B2 is arranged between the third shaft TM3 and thetransmission housing H, such that the third shaft TM3 may be selectivelyconnected with the transmission housing H and act as a fixed element.

The third brake B3 is arranged between the sixth shaft TM6 and thetransmission housing H, such that the sixth shaft TM6 may be selectivelyconnected with the transmission housing H and act as a fixed element.

As described above, although the second clutch C2 is selectivelyconnects the fourth shaft TM4 and the eighth shaft TM8, the fourth shaftTM4 is always directly connected to the input shaft IS such that thesecond clutch C2 may selectively connect the input shaft IS and theeighth shaft TM8 as shown in FIG. 1.

The control elements of the first, second, third, and fourth clutchesC1, C2, C3, and C4 and the first, second, and third brakes B1, B2, andB3 may be realized as multi-plate hydraulic pressure friction devicesthat are frictionally engaged by hydraulic pressure.

FIG. 2 is an operational chart for respective control elements atrespective shift stages in a planetary gear train according to anexemplary embodiment of the present invention.

Referring to FIG. 2, a planetary gear train according to an exemplaryembodiment of the present invention realizes ten forward speeds and fourreverse speeds by operating three control elements among the first,second, third, and fourth clutches C1, C2, C3, and C4 and the first,second, and third brakes B1, B2, and B3 at respective shift-stages.

In the forward first speed shift-stage D1, the third brake B3 and thethird and fourth clutches C1 and C4 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fifth shaftTM5 by the operation of the third clutch C3, and the fifth shaft TM5 isinterconnected with the sixth shaft TM6 by the operation of the fourthclutch C4. In this state, torque of the input shaft IS is input to thefourth shaft TM4, and the sixth shaft TM6 acts as a fixed element by theoperation of the third brake B3, thereby realizing the forward firstspeed and outputting a shifted torque through the output shaft OSconnected with the seventh shaft TM7.

In the forward second speed shift-stage D2, the third brake B3 and thesecond clutch C2 and the fourth clutch C4 are simultaneously operated.

As a result, the fourth shaft TM4 is interconnected with the eighthshaft TM8 by the operation of the second clutch C2, and the fifth shaftTM5 is interconnected with the sixth shaft TM6 by operation of thefourth clutch C4. In this state, torque of the input shaft IS is inputto the fourth shaft TM4 and the tenth shaft TM10, and the sixth shaftTM6 acts as a fixed element by the operation of the third brake B3,thereby realizing the forward second speed and outputting a shiftedtorque through the output shaft OS connected with the seventh shaft TM7.

In the forward third speed shift-stage D3, the second brake B2 and thesecond clutch C2 and the fourth clutch C4 are simultaneously operated.

As a result, the fourth shaft TM4 is interconnected with the eighthshaft TM8 by the operation of the second clutch C2, and the fifth shaftTM5 is interconnected with the sixth shaft TM6 by operation of thefourth clutch C4. In this state, torque of the input shaft IS is inputto the fourth shaft TM4 and the tenth shaft TM10, and the third shaftTM3 acts as a fixed element by the operation of the second brake B2,thereby realizing the forward third speed and outputting a shiftedtorque through the output shaft OS connected with the seventh shaft TM7.

In the forward fourth speed shift-stage D4, the first, second, andfourth clutches C1, C2, and C4 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1, the fourth shaft TM4is interconnected with the eighth shaft TM8 by the operation of thesecond clutch C2, and the fifth shaft TM5 is interconnected with thesixth shaft TM6 by the operation of the fourth clutch C4. In this case,total planetary gear sets PG1, PG2, PG3, and PG4 integrally rotates, anda torque inputted through the fourth shaft TM4 and the tenth shaft TM10is outputted as inputted, thereby forming the forward fourth speed andoutputting the inputted torque to the output shaft OS connected with theseventh shaft TM7.

In the forward fifth speed shift-stage D5, the first brake B1 and thefirst clutch C1 and the second clutch C2 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1, and the fourth shaftTM4 is interconnected with the eighth shaft TM8 by the operation of thesecond clutch C2. In this state, torque of the input shaft IS is inputto the fourth shaft TM4 and the tenth shaft TM10, and the first shaftTM1 acts as a fixed element by the operation of the first brake B1,thereby realizing the forward fifth speed and outputting a shiftedtorque through the output shaft OS connected with the seventh shaft TM7.

In the forward sixth speed shift-stage D6, the first brake B1 and thesecond clutch C2 and the third clutch C3 are simultaneously operated.

As a result, the fourth shaft TM4 is interconnected with the eighthshaft TM8 by the operation of the second clutch C2, and the fifth shaftTM5 is interconnected with the sixth shaft TM6 by the operation of thefourth clutch C4. In this state, torque of the input shaft IS is inputto the fourth shaft TM4 and the tenth shaft TM10, and the first shaftTM1 acts as a fixed element by the operation of the first brake B1,thereby realizing the forward sixth speed and outputting a shiftedtorque through the output shaft OS connected with the seventh shaft TM7.

In the forward seventh speed shift-stage D7, the first brake B1 and thesecond clutch C2 and the fourth clutch C4 are simultaneously operated.

As a result, the fourth shaft TM4 is interconnected with the eighthshaft TM8 by the operation of the second clutch C2, and the fifth shaftTM5 is interconnected with the sixth shaft TM6 by the operation of thefourth clutch C4. In this state, torque of the input shaft IS is inputto the fourth shaft TM4 and the tenth shaft TM10, and the first shaftTM1 acts as a fixed element by the operation of the first brake B1,thereby realizing the forward seventh speed and outputting a shiftedtorque through the output shaft OS connected with the seventh shaft TM7.

In the forward eighth speed shift-stage D8, the first brake B1 and thethird clutch C3 and the fourth clutch C4 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fifth shaftTM5 by the operation of the third clutch C3, and the fifth shaft TM5 isinterconnected with the sixth shaft TM6 by the operation of the fourthclutch C4. In this state, torque of the input shaft IS is input to thefourth shaft TM4, and the first shaft TM1 acts as a fixed element by theoperation of the first brake B1, thereby realizing the forward eighthspeed and outputting a shifted torque through the output shaft OSconnected with the seventh shaft TM7.

In the forward ninth speed shift-stage D9, the first brake B1 and thefirst clutch C1 and the fourth clutch C4 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1, and the fifth shaftTM5 is interconnected with the sixth shaft TM6 by the operation of thefourth clutch C4. In this state, torque of the input shaft IS is inputto the fourth shaft TM4, and the first shaft TM1 acts as a fixed elementby the operation of the first brake B1, thereby realizing the forwardninth speed and outputting a shifted torque through the output shaft OSconnected with the seventh shaft TM7.

In the forward tenth speed shift-stage D10, the first brake B1, thesecond brake B2, and the fourth clutch C4 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1, the fourth shaft TM4is interconnected with the eighth shaft TM8 by the operation of thesecond clutch C2, and the second shaft TM2 is interconnected with thefifth shaft TM5 by the operation of the third clutch C3. In this state,torque of the input shaft IS is input to the fourth shaft TM4, and thefirst shaft TM1 and the third shaft TM3 act as fixed elements by theoperation of the first brake B1 and the second brake B2, therebyrealizing the forward tenth speed and outputting a shifted torquethrough the output shaft OS connected with the seventh shaft TM7.

In the reverse first speed REV1, the third brake B3, the first clutchC1, and the fourth clutch C4 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1, and the fifth shaftTM5 is interconnected with the sixth shaft TM6 by the operation of thesecond clutch C4. In this state, torque of the input shaft IS is inputto the fourth shaft TM4, and the sixth shaft TM6 acts as a fixed elementby the operation of the third brake B3, thereby realizing the reversefirst speed and outputting a shifted torque through the output shaft OSconnected with the seventh shaft TM7.

In the reverse second speed REV2, the third brake B3, the first clutchC1, and the third clutch C3 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1, and the second shaftTM2 is interconnected with the fifth shaft TM5 by the operation of thethird clutch C3. In this state, torque of the input shaft IS is input tothe fourth shaft TM4, and the sixth shaft TM6 acts as a fixed element bythe operation of the third brake B3, thereby realizing the reversesecond speed and outputting a shifted torque through the output shaft OSconnected with the seventh shaft TM7.

In the reverse third speed REV3, the first brake B1, the third brake B3,and the first clutch C1 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fourthshaft TM4 by the operation of the first clutch C1. In this state, torqueof the input shaft IS is input to the fourth shaft TM4, and the firstshaft TM1 and the sixth shaft TM6 act as fixed elements by the operationof the first brake B1 and the third brake B3, thereby realizing thereverse third speed and outputting a shifted torque through the outputshaft OS connected with the seventh shaft TM7.

In the reverse third speed REV3, the first brake B1, the third brake B3,and the third clutch C3 are simultaneously operated.

As a result, the second shaft TM2 is interconnected with the fifth shaftTM5 by the operation of the third clutch C3. In this state, torque ofthe input shaft IS is input to the fourth shaft TM4, and the first shaftTM1 and the sixth shaft TM6 act as fixed elements by the operation ofthe first brake B1 and the third brake B3, thereby realizing the reversefourth speed and outputting a shifted torque through the output shaft OSconnected with the seventh shaft TM7.

As described above, a planetary gear train according to an exemplaryembodiment of the present invention may realize at least ten forwardspeeds and at least four reverse speeds formed by operating fourplanetary gear sets PG1, PG2, PG3, and PG4 by controlling the fourclutches C1, C2, C3, and C4 and the three brakes B1, B2 and B3.

In addition, a planetary gear train according to an exemplary embodimentof the present invention may realize shift stages appropriate forrotation speed of an engine due to multi-stages of an automatictransmission and improve driving stability of a vehicle by utilizing alow rotation speed of an engine.

In addition, a planetary gear train according to an exemplary embodimentof the present invention may maximize engine driving efficiency bymulti-stages of an automatic transmission, and may improve powerdelivery performance and fuel consumption.

In addition, a planetary gear train according to an exemplary embodimentof the present invention may improve reverse performance by realizingshift stages of reverse four speeds

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft for receiving an engine torque; an output shaft for outputting a shifted torque; a first planetary gear set having first, second, and third rotational elements; a second planetary gear set having fourth, fifth, and sixth rotational elements; a third planetary gear set having seventh, eighth, and ninth rotational elements; a fourth planetary gear set having tenth, eleventh, and twelfth rotational elements; a first shaft interconnecting the first rotational element and the fourth rotational element; a second shaft connected with the second rotational element; a third shaft interconnecting the third rotational element and the seventh rotational element; a fourth shaft connected with the fifth rotational element and selectively connected with the second shaft and directly connected with the input shaft; a fifth shaft connected with the sixth rotational element and selectively connected with the second shaft; a sixth shaft interconnecting the eighth rotational element and the twelft rotational element and selectively connected with the fifth shaft; a seventh shaft interconnecting the ninth rotational element and the eleventh rotational element and directly connected with the output shaft; and an eighth shaft connected with the tenth rotational element and selectively connected with the fourth shaft.
 2. The planetary gear train of claim 1, wherein: the first, third and sixth shafts are selectively connected with the transmission housing respectively.
 3. The planetary gear train of claim 1, wherein: the first, second, and third rotational elements of the first planetary gear set are respectively a first sun gear, a first planet carrier, and a first ring gear of the first planetary gear set; the fourth, fifth, and sixth rotational elements of the second planetary gear set are respectively a second sun gear, a second planet carrier, and a second ring gear of the second planetary gear set; the seventh, eighth, and ninth rotational elements of the third planetary gear set are respectively a third sun gear, a third planet carrier, and a third ring gear of the third planetary gear set; and the tenth, eleventh, and twelfth rotational elements of the fourth planetary gear set are respectively a fourth sun gear, a fourth planet carrier, and a fourth ring gear of the fourth planetary gear set.
 4. The planetary gear train of claim 2, further comprising: a first clutch selectively connecting the second shaft and the fourth shaft; a second clutch selectively connecting the fourth shaft and the eighth shaft; a third clutch selectively connecting the second shaft and the fifth shaft; a fourth clutch selectively connecting the fifth shaft and the sixth shaft; a first brake selectively connecting the first shaft and the transmission housing; a second brake selectively connecting the third shaft and the transmission housing; and a third brake selectively connecting the sixth shaft and the transmission housing.
 5. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft for receiving an engine torque; output shaft for outputting a shifted torque; a first planetary gear set having first, second, and third rotational elements; a second planetary gear set having fourth, fifth, and sixth rotational elements; a third planetary gear set having seventh, eighth, and ninth rotational elements; and a fourth planetary gear set having tenth, eleventh, and twelfth rotational elements, wherein the input shaft is directly connected with the fifth rotational element, the output shaft is directly connected with the eleventh rotational element, the first rotational element is directly connected with the fourth rotational element, the third rotational element is directly connected with the seventh rotational element, the fifth rotational element is selectively connected with the second rotational element, the sixth rotational element is selectively connected with the second rotational element, the eighth rotational element is directly connected with the twelfth rotational element and selectively connected with the sixth rotational element and selectively connected with the transmission housing, the ninth rotational element is directly connected with the eleventh rotational element, and the tenth rotational element is selectively connected with the fourth rotational element.
 6. The planetary gear train of claim 5, wherein: the first rotational element, the third rotational element and the eighth rotational element are selectively connected with the transmission housing respectively.
 7. The planetary gear train of claim 5, wherein: the first, second, and third rotational elements of the first planetary gear set are respectively a first sun gear, a first planet carrier, and a first ring gear of the first planetary gear set; the fourth, fifth, and sixth rotational elements of the second planetary gear set are respectively a second sun gear, a second planet carrier, and a second ring gear of the second planetary gear set; the seventh, eighth, and ninth rotational elements of the third planetary gear set are a third sun gear, a third planet carrier, and a third ring gear of the third planetary gear set; and the tenth, eleventh, and twelfth rotational elements of the fourth planetary gear set are respectively a fourth sun gear, a fourth planet carrier, and a fourth ring gear of the fourth planetary gear set.
 8. The planetary gear train of claim 6, further comprising: a first clutch selectively connecting the second rotational element and the fifth rotational element; a second clutch selectively connecting the fifth rotational element and the tenth rotational element; a third clutch selectively connecting the second rotational element and the sixth rotational element; a fourth clutch selectively connecting the sixth rotational element and the eighth rotational element; a first brake selectively connecting the first rotational element and the transmission housing; a second brake selectively connecting the third rotational element and the transmission housing; and a third brake selectively connecting the eighth rotational element and the transmission housing. 